In a conventional electrolytic capacitor, a driving electrolyte solution is soaked into a capacitor element comprising an electrode foil with lead wires leading to the electrodes, which is coiled with intervening separators. This capacitor element is contained in an outer case in the form of a tube with a closed bottom, and a sealer is then mounted on the open end of the outer case. The opening is then sealed by swaging to form the electrolytic capacitor.
However, electrolytic capacitors are now being used at higher ambient temperatures of around 125° C. in automobile components and inverter lighting. The conventional electrolytic capacitor described above cannot withstand long-term use under such high-temperature conditions. That is, during long-term use at a high temperature of 125° C. the electrolytic solution sealed inside the outer case swells the seal rubber, which undergoes thermal oxidation deterioration, thereby reducing the strength and air-tightness of the seal and detracting from the properties of the electrolytic capacitor. Normally, as such an electrolytic capacitor sealer, sealing rubber formed from butyl rubber or ethylene propylene rubber is used.
To resolve these problems, as shown in Patent Documents 1 to 4, it has been proposed that a highly heat-resistant peroxide-cured (crosslinked) butyl rubber be used for the sealer of an electrolytic capacitor. As shown in Patent Document 5, moreover, it has also been proposed that uncrosslinked butyl rubber be crosslinked in the presence of an organic oxide and a polyfunctional monomer having an electron withdrawing group as a means of obtaining highly-heat resistant peroxide butyl rubber.    Patent Document 1: Japanese Patent Application Laid-open No. H9-106932    Patent Document 2: Japanese Patent Application Laid-open No. H11-288853    Patent Document 3: Japanese Patent Application Laid-open No. 2000-173876    Patent Document 4: Japanese Patent Application Laid-open No. 2003-109860    Patent Document 5: Japanese Patent Application Laid-open No. H6-172547
However, although the sealers disclosed in Patent Documents 1 to 4 above have improved properties in comparison with conventional sealers using resin-cured butyl rubber, their performance may not be satisfactory. In particular, although partially crosslinked butyl rubber is sold commercially as suitable butyl rubber for peroxide crosslinking, such rubber has not been suitable for sealers due to problems of workability and the like.
In the method described in Patent Document 5, in which an uncrosslinked butyl rubber is crosslinked in the presence of an organic peroxide and a polyfunctional monomer having an electron withdrawing group, there is a risk of severe reversion, making such rubber unsuitable for sealers, which need to have stable properties.